Page:Proceedings of the Royal Society of London Vol 69.djvu/40

32 the blue-green can be added mthmd being perceived even when the diluted colour is compared directly with the pure (p. 132, 'Colour Measurement ') It can thus easily be believed that much more can be added without appreciable effect when no direct comparison is made with the pure colours.

But probably the best example of the unimportance of the addition of white is given by Mr. Ives's results, where the colours used as primary colours are produced by transmitting white through a red, ^reen, and violet glass respectively; when, as in the examples above, there is transmitted a light which is equivalent to a pure colour with a percentage of white, which in some cases is quite considerable. I confess my surprise at the excellence of the results obtained, which show how exceedingly bad a judge the eye must be of an addition of white to a colour. Provided the quantity that will be left in the printing is not greater than the quantities he obtained by the trans- mission through his glasses and I see no reason why inks should not be produced to secure this the pictures obtainable by the three-colour processes ought not to be inferior to those he obtains by the super- position of the three-coloured transparencies.

We can realise the effect of this addition of white by supposing a coloured picture projected on a screen in a room that is not quite dark. The screen will then reflect, in addition to the colours projected, n certain amount of the diffused light of the room. As this coloured picture would not consist of pure colours undiluted with white, the addition of further white will make the proportion greater than would ccur in printing.

On the other hand, the eye is a very good judge of hue, a very small variation in the proportion of the colours (other than white being easily detected.

Much may be done, by training, to educate the eye to appreciate the relative luminosity of .the colour, or, as it is usually termed, the " value " of the colour, and many artists are able to recognise varia- tions in this almost as easily as in the hue itself. But the average person is not a good judge of this, and it is of much less importance than the hue. For instance, many people fail to see any improvement in a photograph of a landscape or a bunch of flowers taken through an orthochromatic screen, over a photograph of the same subject taken in the usual way.

But circumstances combine to make it difficult to recognise the addition of white even in a picture : the irregularities in the varnishing and the dust upon it are bound to add a proportion of white to the light reflected, and indeed it is very seldom that a picture is so hung that the varnish does not reflect a large amount of white, which we often do not notice until we attempt to take a photograph of it, and then very special lighting is found absolutely necessary.